With the term “high purity propylene (co)polymers” are meant those propylene (co)polymers having a low amount of catalyst residues, also known as “low ash (co)polymers”. For those polymers the total amount of ashes, including Ti, Mg, Cl and Al, is generally lower than 50 ppm.
Propylene polymers with high purity are generally used for film applications and in particular for the production of films for dielectric capacitors. In order to be used for such application the polymers also need to show a medium-broad molecular weight distribution and relatively high cristallinity.
Another field of application for high purity propylene polymers is that of very thin films (around 10 micron) for print lamination, cigarette packing.
As most of the catalysts industrially used are not able to generate polymers with such a low amount of catalyst residues, the propylene polymers intended for the above-indicated applications need to be purified with deashing treatments which, however, make the entire process much more complicated from an operational point of view.
WO2009/077464 and WO2009/077467, for example, describe the preparation of propylene polymers having low ash content carried out in the presence of a catalyst component based, respectively, on diethers and on succinates as internal donors. Those processes are characterized by the use of low Al/Ti molar ratio and the polymers obtained, despite showing a low final content of Al, contain an amount of Cl and Mg that is too high for the intended applications.
It is therefore still felt the need of a process for the production of high purity propylene polymers having a medium-broad molecular weight distribution, high cristallinity and low content of ashes, particularly in terms of Mg and Cl.
The applicant has found a process that can fulfill those needs, by combining the use of a solid catalyst component containing a specific combination of internal donors with the use of a specific amount of aluminum alkyl and with the adoption of a specific temperature range.